MAILBOX: Daylight Drive

Homemade solar hot air collectors on the roofs of the main house and kitchen provide free heat during the wintertime.

A modern DC motor powers a vintage drill press, PV-direct.

Intermediate

I have built a number of houses, and worked a lot with renewable energy systems. Home Power has long been one of my go-to sources for inspiration and information. Over the years, numerous friends have asked me to help them install off-grid systems. Some of those systems are on their third, financially painful battery replacement. It seems that off-grid design has standardized around a very poor energy management approach.

Battery degradation costs of $1,000 per year are common in full-power off-grid systems. But our batteries are costing us about $25 per year. My family lives at Living Energy Farm, a community designed to operate without fossil fuel. Our house stays warm in the winter and cool in the summer. I can surf the ’Net, take a hot shower, or drink a refrigerated glass of lemonade anytime I want. Our power has stayed on for eight years now, without interruption, even as our neighbors lose power over and over again in big storms.

Modern off-grid systems try to imitate AC grid systems by having a robust power supply traveling down a single wire to many uses. We have redesigned our relationship with energy at our farm. There is no good reason to treat solar electricity (PV) like AC power. We have tied many needs for energy directly to specific PV power supplies in a mutlilinear system that is inexpensive, effective, and failproof.

By far, our biggest use of electricity is what we call “daylight drive.” We run high- and low-voltage DC motors directly from PV arrays when the sun is out. Sun comes up, motors run; sun goes down, motors quit. It’s that simple. The amazing thing about DC motors is that they float wonderfully as power input fluctuates. We can’t overload the system. The motors simply speed up and slow down as power fluctuates.

With daylight drive, we cut firewood, process food, and grind grain. We have a fully equipped shop with saws, grinders, a compressor, a drill press, and all of the tools we need to keep the farm running. We also leverage our high-grade energy (electricity) to store lots of low-grade thermal energy. During the day in winter, our daylight-drive heating blowers pull hot air from the cheap, homemade hot air collectors on the roof and pass it under the floor. Heat stored under the floor keeps the house warm when there is no sun.

We can take all the late-night showers we want (because of our solar water collectors and backup wood-fired heating system). Our well pump is daylight drive, and so are the water-heating pumps and our refrigerator. We have a daylight-drive system for charging electronics. A PV module is tied to a $30 voltage regulator that is tied to a half-dozen cigarette-lighter plugs. We use “car chargers” to charge laptops, smartphones, DVD players, and whatever electronic entertainment our guests may bring. Like our other systems, it all floats with the changing weather. None of these systems are dependent on a centralized battery bank.

With most of our electrical load shifted to daylight drive, including basic needs like heating, cooling, and water supply, we store electricity in a centralized battery bank only for DC LED lighting. The problem with conventional design is that it demands powerful, disposable batteries. With a much-reduced need for electrical storage, we are free to use very durable nickel-iron (NiFe) batteries. Miraculously, this ancient battery still works! Since we put in NiFe batteries, our lights have never faded, even when we go through weeks of cloudy weather in the winter. The NiFe batteries tolerate huge voltage swings—and even full discharge—with no damage to the battery. The comparative amp-hour ratings between different kinds of batteries, so we have learned, are essentially meaningless. Our NiFe battery set is seven years old. With little attention, they are showing no signs of fatigue.

We live well with an off-grid design that is cheap, effective, and durable. We hope that in the years to come, we can help more and more people do likewise.

Comments (1)

I think this is one of the more interesting concepts HomePower has printed. Refrigeration or especially freezing is the bigger challenge for storage. Maybe freezing is unnecessary for many foods, and drying might take much of its place especially in a vegan diet. Thanks, Alexis. I'd like to hear more.